CREATE TRIGGER METADATEN_TRG 
BEFORE INSERT ON METADATEN 
FOR EACH ROW 
BEGIN
  <<COLUMN_SEQUENCES>>
  BEGIN
    IF :NEW.METADATEN_ID IS NULL THEN
      SELECT METADATEN_SEQ.NEXTVAL INTO :NEW.METADATEN_ID FROM DUAL;
    END IF;
  END COLUMN_SEQUENCES;
END;
/
